For cutting tools or wear-resistant tools mainly used in the metal cutting process, required by machine industries, useful are WC based hard alloys, various TiC or Ti(CN) based cermet alloys, other ceramics, or high-speed steels.
Among these, a cermet is a sintered body of ceramic-metal composite powder containing, for example, (Ti,W)C, (Ti,W)(CN), TiC, or Ti(CN) and a binder metal, such as Ni, Co, or Fe, as main components, and, as an additive, carbide, nitride, or carbonitride of Group IVa, Va, and VIa metals in the periodic table.
As mentioned above, a cermet is prepared by mixing TiC, Ti(CN), or WC with hard ceramic powder, including Mo2C, NbC, or TaC, and metal powder of Co, Ni, Fe, in a matrix phase, for binding the ceramic powder, and sintering the mixture in a vacuum or in a nitrogen atmosphere.
TiC has very high Vicker's hardness of 3,200 kg/mm2, a considerably high melting point of 3,423 to 3,523 K, and relatively high oxidation resistance until 973 K, and furthermore, has superior properties, including wear resistance, corrosion resistance, electromagnetic radiation properties, and light-collecting properties. Thus, TiC has been mainly used as a high-speed cutting tool material, instead of conventional WC—Co alloys.
In the case where a cermet is prepared using TiC, a binder metal such as Ni is used as a liquid metal upon sintering. In this case, however, a wetting angle becomes greater than WC—Co combinations, undesirably causing rapid grain growth of TiC, leading to decreased toughness.
Nevertheless, a TiC—Mo2C—Ni cermet was first mass-produced by Ford Motor Company, USA, in 1956. Although this cermet was not greatly improved in toughness, it was used in semi-finishing and finishing as high-hardness tool material for precise machining operations.
In the 1960s and 1970s, in order to improve toughness, which is the great disadvantage of the TiC—Ni cermet system, attempts to add various elements to the above TiC—Ni cermet system were made, but did not attain outstanding results.
On the way, in the 1970s, Ti(CN), which is a more stable thermodynamic phase, was realized through the addition of TiC with TiN. Further, because Ti(CN) has a finer structure than TiC, toughness may be improved to some degree, and as well, chemical stability and mechanical impact resistance may be increased.
With the goal of improving toughness, many additive carbides such as WC, Mo2C, TaC, NbC, etc., have been used, and Ti(CN)-M1C-M2C— . . . —Ni/Co type products are commercially available to date.
In the case where carbide is added to improve toughness, a general microstructure of a sintered TiC or Ti(CN) based cermet is observed as a core/rim structure, in which the hard phase of the core/rim structure is enclosed with a binder phase such as Ni or Co.
In the core/rim structure, the core is undissolved TiC or Ti(CN) in the liquefied metal binder (Ni, Co) during sintering, the structure of which has high hardness.
The rim around the core is a solid-solution (which is represented by (Ti, M1, M2 . . . )(CN)) between the core component, TiC or Ti(CN), and the additive carbide, and is a structure having high toughness, rather than high hardness.
In this way, the cermet is advantageous because toughness problems, which are drawbacks of TiC—Ni or Ti(CN)—Ni based simple cermet, are alleviated, thanks to the formation of the rim structure.
However, even if such toughness problems are alleviated, the cermet having the core/rim structure still has a problem in that the toughness is inferior to WC—Co based hard alloys, and thus has not yet completely substituted for WC—Co.
Therefore, lots of effort to develop a cermet having improved toughness through the formation of a complete solid-solution phase having no core/rim structure have been continuously made by tool manufacturers, such as Sumitomo, Mitsubishi, etc.
However, because the amount of the solid-solution phase which is formed during the sintering of composite powder is dependent on the sintering temperature and time, it is impossible to actually obtain a cermet composed exclusively of a complete solid-solution using conventional carbonitride mixture powder or preparation methods thereof.
Therefore, the present inventors have developed solid-solution powder and cermet for substantially achieving a complete solution-solution phase <Korean Unexamined Patent Publication No. 2005-0081553>, and the toughness may be greatly improved according thereto.
However, a cermet that is able to greatly improve toughness even in the case where the complete solid-solution phase is formed as above or a partial solid-solution phase is additionally formed in addition to the complete solid-solution phase and that also has other good properties including high strength, along with a preparation method thereof, has not yet been developed.